Oncology Update: Mast Cell Tumours – Frequently Asked Questions

Oncology Update
Central Toronto Veterinary Referral Clinic
Kevin Finora DVM, Diplomate ACVIM (Oncology and Small Animal Internal Medicine)
Mast Cell Tumours – Frequently Asked Questions

What is a MCT? A MCT is a common round cell cancer, or cancer of the immune system. It typically localizes to the skin. It is the most common canine skin cancer. MCT, in some cases, appear as a disease involving the internal organs. This form of the disease is far more aggressive than that restricted to the skin. Breeds most at risk for cutaneous MCT include Boxers and Pugs.

What is grade? Grade is determined by histological criteria and assesses tumour cell differentiation, mitosis, mitotic index and necrosis. Grade is associated with prognosis and is the most prognostic survival indicator we have available. There are two Grading Systems; the Patnaik system uses, three grades- I, II and III. Grade I has the best prognosis and Grade III is the worst. The MSU Grading System uses two grades, Low and High, with High having the worst prognosis.

How are MCT treated? Local control is the hallmark of therapy. Control involves surgery, radiation therapy or a combination. I prefer the surgical option, when possible. Due to the aggressive nature of the tumour, wide margins are necessary to have the best chance to obtain complete control. Surgical margins of 3cm are considered standard. Though one study indicated in less aggressive MCT, 2cm margins will completely remove the tumour about 90% of the time. Aggressive surgery is always best as “the first surgery is the best chance for cure.” Histologic margins of 5mm are needed to declare the margins clear. Excision with less than 5mm margins is considered marginal and incomplete.

What can be done if the surgical margins aren’t complete? This is the toughest question to know how to answer. Additional local control is generally recommended – absolutely for Grade III or High Grade tumours, and may involve more surgery or radiation therapy. Recurrence rates may be as low at 23% for incompletely resected Grade II MCT, but 71% will recur within 1 year. I recommend additional local control. For Grade I or Low Grade tumours, if margins are incomplete watchful waiting can be considered, if the client is against further local control, or if it is not possible. If a low grade tumour has a low proliferation index, we can feel more confident in perhaps electing to monitor for new or recurrent disease.

Does chemotherapy play a role in MCT therapy? Chemotherapy’s role is adjunctive (given after local control) and is meant to address or prevent the metastatic aspect of MCT. Chemotherapy can improve outcomes with response rates as high as 78%. Grade I tumours generally don’t require chemotherapy while Grade III tumours always do. What about Grade II MCT? A recent study demonstrated certain proliferation markers (AgNOR, Ki67) are related to Grade. I recommend all Grade II (Patnaik system) and ALL MUS System MCT have a proliferation panel completed and those with intermediate or high markers be treated with chemotherapy.

What about c-kit and Palladia? C-kit is a growth factor which is expressed and often mutated in canine MCT. Palladia can inhibit the c-kit growth factor and has been demonstrated to be of some use in the treatment of canine MCT. The use of Palladia is considered a targeted therapy.

What is the prognosis? The outcome is very Grade dependant. With appropriate local control 93% of dogs will be alive at 3 years. The largest study looked a survival rates at 1500 days with surgical treatment only. Eighty-three per cent of Grade I dogs were alive at 1500 days, 44% of Grade II and 6% of Grade III. Dogs with High Grade tumours tend to live less than one year.

Dr. Kevin Finora is a Board Certified Oncologist and Small Animal Internist who is part of the Healthcare Team at the Central Toronto Veterinary Referral Clinic. He is available for referrals and consultations Monday to Thursday (including Monday and Tuesday evenings). Please contact him with any oncology questions or concerns.

Emergency Veterinarian Toronto

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